Terpenes are hydrocarbons commonly produced by a wide variety of plants but also found in many other organisms, including bacteria, fungi, algae, insects, and some larger animals. Terpenes are assembled by a terpene synthase from five-carbon building blocks called isoprenes (C5H8) that can be joined end-to-end in linear chains or can be arranged to form rings.

Terpenes are useful in many commercial applications, including biofuels, solvents, pharmaceuticals, cosmetics, fragrances, essential oils, resins, pigments and colorants, anti-microbial and anti-fungal agents, flavorings, nutraceuticals, and medicaments. Although terpenes can be extracted from natural sources, extraction is problematic for commercial applications due to low yields, difficulties in large-scale cultivation of the natural sources, and, in some cases, toxicity of the solvents required for extraction. Chemical synthesis of terpenes is also difficult and costly. Thus, there is a need for alternative sources of terpenes, including ways to manufacture them on a commercial scale. Reports describe microbial strains that have been engineered to produce terpenes. See Martin et al., (2003) Nature Biotech. 21:796-802; U.S. Pat. No. 6,818,424; Yuba et al. (1996) Arch Biochem Biophys 332:280-287 (reporting limonene synthase from Perilla frutescens expressed in E. coli)).
Certain plants and microorganisms use geranyl diphosphate (“GPP”) as a ten-carbon phosphorylated terpene precursor. In one biosynthetic pathway, GPP is produced from the five-carbon precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). Prenyl transferases (e.g., GPP synthases) are described in, e.g., Schmidt & Gershenzon, Phytochemistry 69:49-57 (2008); Bouvier et al., The Plant Journal 24(2):241-252 (2000); Burke & Croteau, Archives of Biochemistry and Biophysics 405:130-136 (2002); Burke et al, PNAS, 96(23):13062-13067 (1999); U.S. Pat. No. 6,303,330; Vandermoten et al, FEBS Letters 582:1928-1934 (2008).
GPP can combine with IPP to make the fifteen-carbon phosphorylated terpene precursor farnesyl diphosphate (FPP). Typically, both steps in this scheme—IPP/DMAPP to GPP and GPP to FPP are facilitated by a single enzyme: an FPP synthase, such IspA from E. coli.

Because IspA preferentially produces FPP (C15) compared to GPP (C10), this pathway likewise leads to preferential production of terpenes having fifteen or more carbon atoms rather than monoterpenes having only ten carbon atoms.